Ok so, I'm largely in the same boat as you (selective school, year 11, trying to get into MX2).
If you went to Rui's lecture yesterday (bloody brilliant may I just add) one of the first things he said was if you do MX2 you need to try it out first ie. don't drop before half-yearlies. I reckon that's a pretty damned good idea, and that if you've already made it in, it's worth a shot.
However, if you find the workload is still too much, you could drop down to 3U or 2U and still have your 10 units.
Modern is a bloody killer in terms of content, extremely rewarding even though it makes me want to die sometimes, and I can see where your enjoyment comes from. Similar thing with bio, or so I've heard since I don't do it. Either way, since you absolutely love these subjects never drop
Otherwise, if the workload is too much maybe drop chem? I think if you can't cope drop chem or MX2.
Having said that though, that's just f you want to drop a subject. You can still spend time on your other content heavy subjects, and study every day just a little on MX2 and still succeed in it. People I know have done this and have gotten high marks for MX2 as well as their content heavy subjects, and it is doable. I guess it just depends on how this pans out for you and how you feel about the workload.
Hope this helps